The present invention relates to coating a steel pipe having a welding seam; but more particularly, the invention relates to coating such a pipe or tube by means of a thermoplastic such as polyethylene.
Coatings are applied to pipes or tubes, for example, by means of tapes, ribbons, or a hose, placed onto and around the tube which has been covered previously with an adhesive. Such a protective coating is needed, for example, when the pipes or tubes are installed underground.
A pipe or tube may be coated or covered, for example, by extruding the thermoplastic onto the pipe, or in some other fashion.
More specifically, German Pat. No. 17,71,764 describes a method in which a thermoplastic ribbon is helically wrapped around a rotating, heated steel pipe, together with an adhesive tape. Both ribbons are separately extruded, just prior to wrapping. A problem may arise here if the tube or pipe has a radially outwardly protruding bead of a welding seam. The ribbons may not fully adhere to the seam and/or the adjoining areas so that narrow, capillary-like gaps remain. Moreover, these gaps may form a long, continuous gap along the seam. Moisture may penetrate and migrate for the length of the pipe and corrode it. The problem arises, regardless whether the seam extends axially or helically. The problem exists also when the coating is applied as an in situ extruded hose.
Another problem arises in coating or covering the increase in wear of the cover above the protruding seam. It was found that, indeed, the cover is or will become thinner on the seam. This can be counteracted by extruding, or otherwise applying, the coating or cover to be thicker than actually necessary. Obviously, this is to some extent a waste of material because the major part of the surface of the pipe is now covered to an excessive degree.
The latter problem has been dealt with in U.S. Pat. No. 3,814,646, see also German Pat. No. 21,17,618 in that strip sections are placed on the seam (only) just prior to wrapping a regularly thick ribbon around the pipe. This patent describes a particular placement mechanism for such a strip, being actuated once for each full turn of the pipe, and the length of that strip is equal to the rate of axial progression of the pipe per revolution.
German Pat. No. 24,07,427 describes another method in which an extruded and stretched ribbon is helically wrapped around the (rotating and axially advancing) pipe. Whenever a section of the seam is about to be covered, the stretch force is diminished. Thus, the cover will be thicker on the seam than elsewhere. Moreover, the formation of gaps adjacent to the seam is prevented by the use of a pressure roller, acting on the freshly laid-on ribbon portion just behind the seam. This method is quite suitable, but there is a need for reenforcing the coverage of a welding seam which is not dependant upon the wrapping of ribbon for the principal cover. Also, there is a need to provide for such supplemental coverage in cases in which the main wrapping machine is not provided with means for changing the tension.